7-Hydroxy coumarin-3-carboxylic acid (7-HCCA) is a photoacid that exists in three distinct forms: neutral, anionic and di-anionic. It falls under the category of hydroxy aromatic and carboxy aromatic molecules. This molecule exhibits strong fluorescence and is commonly employed in the detection of hydroxyl radicals. In our study, we utilised absorption and fluorescence spectroscopy to investigate the photophysical behaviour of 7-HCCA in different micellar systems. Through our experiments, we observed that as we varied the pH within the ranges of 2-3, 4-6, 7-10, the three aforementioned forms of 7-HCCA are emerged. Specifically, in cationic surfactants, the dye displayed an equilibrium between two forms of the 7-HCCA in the ground state at any pH. Furthermore, our steady-state excitation studies also proved the presence of an equilibrium between two forms of the 7-HCCA in all cationic systems. By examining the steady-state emission spectra, we also observed the presence of two distinct species in the excited state. When cationic surfactants were introduced, we observed a decrease in the fluorescence intensity and quantum yield compared to that observed in the water medium. It was evident that the photophysical properties of 7-HCCA were modulated with variations of the chain length and head group of the surfactants. In summary, our investigation shed light on the photophysical characteristics of 7-HCCA in different micellar systems with varying the pH. By employing the absorption and fluorescence spectroscopy, we successfully identified the coexistence of multiple forms of 7-HCCA and observed how the cationic surfactants influenced its fluorescence emission properties, ultimately contributing to a deeper understanding of its behaviour in these systems.

7-羟基香豆素-3-羧酸 (7-HCCA) 是一种光酸，以三种不同的形式存在：中性、阴离子和双阴离子。它属于羟基芳香族和羧基芳香族分子的范畴。该分子表现出强烈的荧光，通常用于羟基自由基的检测。在我们的研究中，我们利用吸收光谱和荧光光谱来研究 7-HCCA 在不同胶束系统中的光物理行为。通过我们的实验，我们观察到当我们在 2-3、4-6、7-10 范围内改变 pH 值时，就会出现上述三种形式的 7-HCCA。具体来说，在阳离子表面活性剂中，染料在任何 pH 值下均表现出两种基态 7-HCCA 形式之间的平衡。此外，我们的稳态激发研究还证明了所有阳离子系统中两种形式的 7-HCCA 之间存在平衡。通过检查稳态发射光谱，我们还观察到激发态下存在两种不同的物质。当引入阳离子表面活性剂时，我们观察到与在水介质中观察到的荧光强度和量子产率相比，荧光强度和量子产率有所下降。很明显，7-HCCA 的光物理性质随着表面活性剂的链长和头基的变化而调节。总之，我们的研究揭示了 7-HCCA 在不同 pH 值的胶束系统中的光物理特性。通过采用吸收光谱和荧光光谱，